1. Field of the Invention
The present invention relates generally to an apparatus and method for delivering fuel for combustion in an internal combustion engine. More specifically, the present invention relates to an apparatus and method for supplying and controlling an input parameter to a pulse type fuel injector.
2. Description of the Related Art
An internal combustion engine ignites a mixture of air and combustible fuel within one or more combustion chambers to provide rotational motive force, or torque, to do work. Along with many other factors, proper operation of an internal combustion engine is dependent upon an adequate supply of fuel for combustion. Two measures of engine performance are illustrative of this dependency: engine torque and engine speed. Generally, the torque produced is proportional to the volume of fuel efficiently combusted during a given combustion cycle. The greater the volume of fuel combusted the greater the force produced from the combustion.
For most applications an engine must be able to provide torque at a range of speeds. Engine speed is generally a function of the flow rate of fuel to the combustion chamber. Increasing the speed of the engine shortens the duration of each combustion cycle. Thus, a fuel delivery system must provide the desired volumes of fuel for each combustion cycle at increasingly faster rates if the engine speed is to be increased. Moreover, engine torque and speed can both be limited by the fuel delivery system. Engine torque can be limited by an inability to supply the engine with a sufficient volume of fuel for the combustion cycle. Alternatively, engine speed can be limited by the inability to supply the required volumes of fuel at a desired rate.
In addition to combustible fuel, oxygen is also necessary for combustion. There are various methods of providing fuel and oxygen for combustion to a combustion chamber. The surrounding air, typically, acts as the source of oxygen. An air intake draws in the surrounding air to mix with the fuel. Some delivery systems mix the air and fuel before the two substances are delivered to the combustion chamber. Alternatively, the fuel and air can be delivered separately and mixed within the combustion chamber. Some systems use carburetors to draw fuel vapor into an air stream that is then fed into the combustion chamber. Still other systems use fuel injection to produce fuel vapor from a liquid fuel spray.
There are many current systems and methods of fuel injection. Typically, a programmable logic device controls the operation of the fuel injection system. One or more pumps are used to produce a source of pressurized fuel. A fluid actuator, typically a solenoid operated valve, initiates a flow of pressurized fuel to an injection nozzle. In some applications the fluid actuators produce a surge in fuel pressure. The surge in pressure of the fuel causes the injection nozzle to open, allowing pressurized fuel to flow through the injection nozzle. The shape of the outlet of the injection nozzle contributes to the atomization of the fuel as it exits the injection nozzle. Still other fuel injection systems use an integrated pump and injection nozzle assembly. The pump is electrically operated and controlled to deliver desired volumes of pressurized fuel at desired rates.
Direct fuel injection is a method of fuel injection in which liquid fuel under pressure is injected directly into a cylinder before combustion is initiated in the cylinder by a spark plug. The fuel injection system converts the liquid fuel into an atomized fuel spray. The atomization of the liquid fuel increases the amount of fuel vapor produced. Increasing the amount of fuel vapor is important because it is the ignition of the fuel vapor that produces the combustion in the cylinder. Increasing the pressure of the fuel will also increase the atomization of the fuel when injected into a cylinder.
The available fuel volumes and flow rates for a given fuel delivery system are limited. Typically, the fuel delivery system will be sized to provide adequate fuel volumes and flow rates for the normal expected range of engine operation. However, the fuel delivery system may be increasingly unable to supply the desired fuel volumes at the desired rate at higher engine speeds. Thus, it may arise that the available engine torque and speed may be limited by the ability of the fuel delivery system to supply fuel for combustion. This is particularly the case when fuel delivery systems for one type of engine are applied to higher performance engines, with correspondingly higher fuel volume and flow rate requirements dictated by higher torque, speed and power capabilities.
Another source of limitation in fuel delivery systems is found in the injectors' cycle time. Cycle time refers to the amount of time required for a fuel injector to load with fuel, discharge the fuel into the combustion chamber and then return to its original position to start the cycle over again. Cycle time is typically short for fuel injectors. For example, injectors used in a direct injection system can obtain a cycle time of 0.01 seconds. That equates to the injectors being able to load with fuel, discharge the fuel into the combustion chamber, and then prepare to reload for a subsequent cycle 100 times in a single second. While this cycle time seems very short, it is often desirable to reduce this time even further when possible.
Reduction of cycle time is desirable for several reasons. First, cycle time contributes to a number of engine performance characteristics including low speed torque and high speed power. By reducing the cycle time of the fuel injectors, these two engine performance characteristics can be improved. Second, in certain applications a small window of variability is found to be associated with cycle time. This window of variability is a short period of time which is only a small fraction of the entire cycle time. However, during this short period of time, the variability causes the fuel injectors to discharge either slightly prematurely, or slightly delayed relative to a target discharge time. Having the injector actually discharge at the target discharge time is important for producing efficient power and torque. The target discharge time is determined as a function of various parameters, one of which is the corresponding timing of a spark plug being fired inside the combustion chamber for the ignition of the fuel vapor. If the fuel injection is either premature or delayed, improper combustion will occur resulting in unburned fuel and decreased engine output. The ability to design and produce internal combustion engines having more predictable and controlled performance characteristics is dependent, in part, on being able to address issues such as faster cycle times and reduced injector discharge variability.